eLFH - Physiological changes during Pregnancy Flashcards
Usual weight gain during pregnancy
10 to 20 kg
Recommended weight gain for women with normal pre-pregnancy BMI
11.5 to 16 kg
Cause of weight gain during pregnancy
Foetal growth
Placenta
Amniotic fluid
Uterus
Breasts
Fat
Blood
Extravascular extracellular fluid
Cause of aortocaval compression
Weight of gravid uterus compress great vessels against lumbar vertebral bodies
(IVC > Aorta)
Consequence of aortocaval compression
IVC obstruction causes fall in venous return to heart leading to drop in maternal cardiac output and BP
Common alternative name for aortocaval compression
Supine hypotension
Symptoms of aortocaval compression
Dizzy
Nausea
Therefore women usually learn to avoid this position
Consequence if aortic compression predominates aortocaval compression
Maternal BP (measured above level of compression) will be normal or raised
However blood supply to uterus and fetoplacental unit (originates below level of compression) reduces and compromises foetus without maternal hypotension
Azygous venous system
Internal vertebral venous plexus around spinal cord
Consequence of IVC compression on azygous venous system
Azygous venous system becomes dilated with IVC compression
Causes engorgement of veins within the spinal canal
Degree of tilt to left side required to reliably avoid aortocaval compression
15 degrees is the compromise
Realistically need 30 degrees
Full left lateral position impractical for obstetric procedures
Gestation from which left lateral tilt is required
20 weeks onwards
Approximate uterine blood flow at term
700 ml/min
Blood supply to the uterus
Graph of changes to heart rate, stroke volume and cardiac output during pregnancy with weeks gestation
Why does cardiac output increase further during labour and by how much
CO increases by further 40% due to pain
Why does cardiac output increase immediately following delivery
Autotransfusion of blood from the uterus
Caution with autotransfusion of blood from uterus
If high risk of fluid overload then can precipitate this from point of delivery to around 48 hours after
Volume of blood autotransfused from uterus to mother as uterus contracts
~ 500 ml
Average resting heart rate during pregnancy
85 bpm
Changes to blood pressure during pregnancy
Systolic and diastolic pressure fall (diastolic more so than systolic) and then increases back to pre-pregnancy BP by term
Gestation at which BP is at its lowest
20 weeks
Cause for drop in BP during pregnancy
Fall in systemic vascular resistance
MAP = CO x SVR
Spirometry trace of non-pregnant vs pregnant adult
Lung volumes which are increased in pregnancy
Tidal volume
Respiratory rate
Minute ventilation
Lung volumes which are reduced in pregnancy
Functional residual capacity
Expiratory reserve volume
Residual volume
Total lung capacity
Why does pregnancy cause faster fall i PaO2 during apnoea
Reduced FRC
Higher oxygen demands
Recommended pre-oxygenation method for pregnancy GA
3 minutes tidal breathing
Why does end tidal oxygen concentration rise faster in pregnant women
Higher minute ventilation in pregnancy
When does PaCO2 decrease during pregnancy and implication
Early in pregnancy
Often lower PaCO2 present in women anaesthetised for termination of pregnancy or ectopic pregnancy
Changes to ventilator settings for term LSCS under GA
Slightly higher RR and VT
Targeting lower EtCO2 / PaCO2 of 4.1
Alveolar gas equation
Why does PaO2 rise as PaCO2 falls during pregnancy
Alveolar partial pressure O2 increases as per alveolar gas equation
Therefore higher diffusion gradient into arterial blood
Cause for more difficult intubation in pregnancy
Mucosa more vascular and oedematous
Subtly alters laryngoscopy views
Reason to avoid nasal intubation, NG tube or suction to nose during pregnancy
Increased vascularity of mucosa increases chance of haemorrhage
Changes to contents of chest during pregnancy
Diaphragm and chest contents displaced cephalad as pregnancy advances
AP diameter of chest increases to accommodate this
Changes to carina during pregnancy
Carina displaced cephalad as pregnancy advances
Therefore shorter distance from teeth to carina and higher chance of endobronchial intubation
Changes to heart during pregnancy
Heart is enlarged and pulmonary vessel engorgement
ECG changes during pregnancy
Left axis shift
T wave inversion in V2
GI changes during pregnancy
Gastro-oesophageal reflux
Reasons for increased gastro-oesophageal reflux during pregnancy
Lower oesophageal sphincter tone reduced
After 20 weeks reflux promoted by gravid uterus
Increased gastric contents volume
Changes to gastric contents during pregnancy
pH falls (3.0 -> 2.4)
Volume increases (0.24 -> 0.49 ml/kg)
Risks of GA which are increased in pregnancy
Gastric contents aspiration
Hypoxaemia
Death
(Due to GI changes and reduced time for airway manipulation)
Methods to reduce risks from GI changes
Reduce gastric acidity
RSI
Gestation from which all pregnant women should have RSI and considered high risk of aspiration
All women over 16 - 18 weeks should have RSI
Prior to 16 weeks gestation, if there are no other risk factors, then RSI is not mandatory
How soon after delivery is RSI still mandatory for GA
Withing 48 hours of delivery RSI is mandatory
After 48 hours, risk of gastric contents returns to pre-pregnancy levels
Haematological changes in pregnancy
Plasma volume rises
RBC volume increases
Hypercoagulability
Why does Hb concentration fall during pregnancy
RBC volume increase to compensate for blood loss at delivery, but not as much as plasma volume increases, therefore [Hb] falls
Lower limit of normal Hb concentration for pregnant women
110
Cause of hypercoagulability in pregnancy and therefore raised VTE risk
Raised plasma levels of:
- Fibrinogen
- Factor VII
- Factor X
- Factor XII
Decreased fibrinolysis
Neurological changes in pregnancy with implication on anaesthesia
Reduced doses of drugs are required to induce and maintain general and regional anaesthesia
MAC for inhaled agents typically reduced by ~30%
From late first trimester onwards local anaesthetic dose typically reduced by ~30% for regional anaesthesia
Induction dose for IV agents also reduced
Renal changes in pregnancy
Rise in:
- GFR
- Kidney size
- Urinary collecting system size
- Glycosuria
- Proteinuria
Amount GFR rises in pregnancy
50-70% increase
Cause for increased glycosuria in pregnancy
Rise in GFR can overwhelm capacity for tubular re-absorption of glucose
Why are UTIs and pyelonephritis more common in pregnancy
Dilated urinary collecting system (renal pelvis + calyces + ureters) leads to tendency of urine stasis
Also increased glycosuria
Upper limit of normal proteinuria level in pregnancy
300 mg/24 hr collection
Changes to plasma albumin concentration in pregnancy
Albumin concentration falls in early pregnancy and is then static up to term
Mean urea concentration in pregnancy
3.3 mmol/L
Mean creatinine concentration in pregnancy
51 micromol/L
Hormones which change in pregnancy
Progesterone
Human chorionic gonadotrophin (hCG)
Human placental lactogen (hPL)
Aldosterone
Progesterone changes in pregnancy
Concentration rises to a peak near term
Falls just before term - may contribute to initiation of labour
Progesterone responsible for many of the physiological changes of pregnancy
Vital for maintenance of pregnancy
Alternate name for hPL (human placental lactogen)
Chorionic somatomammotrophin
hPL features
Peptide hormone
Structure and function similar to growth hormone
Consequence of raised hPL
Causes impaired glucose tolerance due to insulin resistance
Aldosterone changes in pregnancy
Aldosterone secretion from adrenal cortex increases
Results in Na+ and water retention
Metabolic changes in pregnancy
Rise in maternal basal metabolic rate
Also contributes to increased oxygen consumption
hCG features
Peptide hormone
Produced by embryo and later by placenta
Role of hCG in pregnancy
Maintain the corpus luteum and hence maintain progesterone production
Possible has a role in the altered immune system in pregnancy
